Method for electrodepositing a dark chromium layer on a substrate and substrate having at least one side fully covered with a dark chromium layer

ABSTRACT

The present invention relates to a method for electrodepositing a dark chromium layer on a substrate and a substrate having at least one side fully covered with a dark chromium layer. The method includes utilizing an aqueous trivalent chromium electroplating bath comprising colloidal particles and a step of treating the substrate with a rinse liquid having a temperature of 50° C. or more.

FIELD OF THE INVENTION

The present invention relates to a method for electrodepositing a dark chromium layer on a substrate and a substrate having at least one side fully covered with a dark chromium layer. The method includes utilizing an aqueous trivalent chromium electroplating bath comprising colloidal particles and a step of treating the substrate with a rinse liquid having a temperature of 50° C. or more.

BACKGROUND OF THE INVENTION

From the very beginning of chromium layers an interest in dark chromium layers was observable. Beginning with dark hexavalent chromium layers, the focus today significantly shifted to trivalent chromium layers due to a higher environmental acceptance.

Typically, each chromium layer, in particular dark chromium layers, are characterized by color values referenced to the L*a*b* color-space system. While the value L* defines the brightness (or sometimes also referred to as lightness), values a* and b* define the color of a respective chromium layer. While a L* value of 100 defines a diffuse white, a L* value of 0 is a deep black. Values for a* and b* can be positive and negative, wherein a* values describe the colors green (negative) and red (positive), while b* values describe the colors blue (negative) and yellow (positive). A combination of a* and b* with 0 describes a neutral grey color, turning into a deep neutral black the lower the L* value (e.g. of 50 or below).

Depending on the chemical composition of a respective aqueous trivalent chromium electroplating bath and/or its deposition parameters a huge variety of L*a*b* values can be generated. The more pronounced these variations are the more obvious is an optical difference between two electroplated chromium layers.

It has been observed in the recent past that in particular dark chromium layers experience a certain color instability over time. This means that directly after depositing the dark chromium layer has certain L*a*b* values which are at least slightly shifting over time the next days and weeks. As a result, an undesired color change or at least perception occurs. In most cases, this is inacceptable for many decorative applications.

It has been also observed that after a sufficiently long time this change comes to an end. In other words, after a sufficient time, which can last for weeks or even month, the color becomes stabilized and does not undergo further significant changes. Sadly, from the commercial point of view it is not acceptable to store respective items for such a long time prior to its final utilization. Therefore, an accelerated aging or artificial aging has been tested.

US 2020/094526 A1 refers to a black plated resin part and method producing the same. It discloses various aging tests including an immersion in warm water at 80° C. for a comparatively short time.

Although such an accelerated aging provides acceptable results and a certain color stability after a comparatively short time, own experiments have shown that the color homogeneity/distribution over an entire area often is not sufficient; in particular if various current densities were locally involved in the deposition process. As a result, a homogeneous color perception can be disturbed, which is particularly not acceptable in decorative applications.

There is therefore an ongoing demand to provide improved methods and electroplating baths for providing improved dark electroplated chromium layers.

Objective of the Invention

It was furthermore an objective of the present invention to provide a method for electrodepositing a dark chromium layer on a substrate, which overcomes the aforementioned disadvantages, particularly allowing a more homogeneous color perception and therefore an increased color homogeneity without compromising a relatively quick accelerated aging.

It was also an objective of the present invention to provide a respective substrate having such characteristics.

SUMMARY OF THE INVENTION

These objectives are solved by the present invention, i.e. by a method for electrodepositing a dark chromium layer on a substrate, the method comprising the steps

-   -   (a) providing a substrate,     -   (b) providing an aqueous trivalent chromium electroplating bath         comprising         -   (i) trivalent chromium ions,         -   (ii) one or more than one complexing agent for said             trivalent chromium ions,         -   (iii) colloidal particles, and         -   (iv) one or more than one sulfur-containing compound having             a sulfur atom with an oxidation number of +5 or below,     -   (c) contacting the substrate with said electroplating bath and         applying an electrical current such that the dark chromium layer         is electrolytically deposited on the substrate and is fully         covering at least one side of the substrate,     -   (d) treating the substrate comprising the electrolytically         deposited dark chromium layer with a rinse liquid, wherein the         rinse liquid has a temperature of 50° C. or more and wherein         during step (d) no electrical current is applied.

In the context of the present invention, emphasis is given on decorative applications. Thus, in the context of the present invention, the dark chromium layer preferably refers to decorative dark chromium layer. Therefore, the method of the present invention focuses on substrates, which have a dark chromium layer fully covering at least one side of said substrate. In other words, the dark chromium layer is at least fully covering a first side of said substrate. Most preferably this is a front side, which means in the context of the present invention that this side is the visually/optically perceived side of the substrate. In most cases it is preferably a decorative substrate, designed for being visually perceived by an end-user. Preferably, optionally in addition at least one more side is also fully covered by the dark chromium layer. If more than one side is fully covered by the dark chromium layer (i.e. a second, third, etc. side), most preferably the side having the larger surface area (and preferably being the visually perceived one by the end-user) is focused on in the context of the present invention. This preferably also applies to the substrate according to the present invention.

As shown below in the example section, substrates having at least one side fully covered by the dark chromium layer exhibit an improved color homogeneity (i.e. a reduced ΔE), thereby reducing the risk of homogeneous color perception disturbance.

DETAILED DESCRIPTION OF THE INVENTION

In step (a) of the method of the present invention, a substrate is provided.

Preferred is a method of the present invention, wherein the substrate comprises a non-metallic substrate, preferably comprises a plastic substrate.

Preferred is a method of the present invention, wherein the non-metallic substrate, preferably the plastic substrate, comprises acrylonitrile butadiene styrene (ABS), acrylonitrile butadiene styrene—polycarbonate (ABS-PC), polypropylene (PP), polyamide (PA), polyurethane (PU), polyepoxide (PE), polyacrylate, polyetherimide (PEI), a polyetherketone (PEK), mixtures thereof, and/or composites thereof; preferably acrylonitrile butadiene styrene (ABS), acrylonitrile butadiene styrene—polycarbonate (ABS-PC), polyamide (PA), polyurethane (PU), polyepoxides (PE), polyacrylate, mixtures thereof, and/or composites thereof. Such plastic substrates are typically used in decorative applications such as automotive parts, in particular ABS and ABS-PC.

Preferred is a method of the present invention, wherein the substrate, preferably the non-metallic substrate, more preferably the plastic substrate, comprises at least one metal layer (most preferably in addition). Preferably, the at least one metal layer is selected from the group consisting of a copper layer, a copper alloy layer, a nickel layer, a nickel alloy layer, and combinations thereof.

Preferred is a method of the present invention, wherein after step (a) and prior to step (c) the method of the present invention includes step

-   (a1) pre-treating the substrate, preferably cleaning the substrate,     most preferably degreasing and/or pickling the substrate.

Preferably, the degreasing comprises an electrolytic degreasing.

Preferably, the pickling comprises a contacting with an acid, preferably an inorganic acid.

Step (a1) is preferably followed by a water rinse.

In step (b) of the method of the present invention, said aqueous trivalent chromium electroplating bath is provided.

Said electroplating bath comprises water, preferably at least 55 vol.-% or more is water, based on the total volume of the electroplating bath, more preferably 65 vol.-% or more, even more preferably 75 vol.-% or more, yet even more preferably 85 vol.-% or more, still more preferably 90 vol.-% or more, most preferably 95 vol.-% or more. Most preferably, water is the only solvent.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is acidic, preferably having a pH ranging from 1.5 to 5.0, more preferably from 2.1 to 4.6, even more preferably from 2.4 to 4.2, yet more preferably from 2.7 to 4, most preferably from 3.0 to 3.8. The pH is preferably adjusted with hydrochloric acid, sulfuric acid, ammonia, potassium hydroxide, and/or sodium hydroxide.

The aqueous trivalent chromium electroplating bath comprises (i), trivalent chromium ions.

Preferred is a method of the present invention, wherein in said electroplating bath the trivalent chromium ions have a total concentration ranging from 5 g/L to 35 g/L, based on the total volume of the electroplating bath, preferably from 6 g/L to 32 g/L, more preferably from 7 g/L to 29 g/L, even more preferably from 8 g/L to 26 g/L, yet even more preferably from 9 g/L to 23 g/L, most preferably from 10 g/L to 22 g/L.

Preferably, the trivalent chromium ions are from a trivalent chromium salt, preferably from an inorganic chromium salt and/or an organic chromium salt, most preferably from an inorganic chromium salt. A preferred inorganic chromium salt comprises chloride and/or sulfate anions, preferably sulfate anions. A very preferred inorganic chromium salt is basic chromium sulfate. A preferred organic chromium salt comprises carboxylic acid anions, preferably formate, acetate, malate, and/or oxalate anions.

Preferred is a method of the present invention, wherein in the aqueous trivalent chromium electroplating bath the trivalent chromium ions together with optional iron ions (regarding iron ions, which are optional but preferred in some cases, see text below) represent 80 mol-% or more of all transition metal ions in the aqueous trivalent chromium electroplating bath, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably 90 mol-% or more, more preferably 93 mol-% or more, even more preferably 96 mol-% or more, most preferably 98 mol-% or more.

The aqueous trivalent chromium electroplating bath comprises (ii), one or more than one complexing agent for said trivalent chromium ions.

Such compounds keep the trivalent chromium ions in solution. Preferably, the one or more than one complexing agent is not a compound of (iv) and is therefore preferably different from (iv).

Preferred is a method of the present invention, wherein in the aqueous trivalent chromium electroplating bath the one or more than one complexing agent comprises an organic acid and/or salts thereof, preferably an organic carboxylic acid and/or salts thereof, most preferably an organic carboxylic acid comprising one, two, or three carboxylic groups and/or salts thereof.

The organic carboxylic acid and/or salts thereof (preferably also said organic carboxylic acid comprising one, two, or three carboxylic groups and/or salts thereof) are preferably substituted with a substituent or unsubstituted. A preferred substituent comprises an amino group and/or a hydroxyl group. Preferably, the substituent does not comprise a SH moiety and/or a SCN moiety.

More preferably, the organic carboxylic acid and/or salts thereof (preferably also said organic carboxylic acid comprising one, two, or three carboxylic groups and/or salts thereof) comprise amino carboxylic acids (preferably alpha-amino carboxylic acids), hydroxyl carboxylic acids, and/or salts thereof. Preferred (alpha-) amino carboxylic acids comprise glycine, aspartic acid, and/or salts thereof. Preferably, the amino carboxylic acids (preferably alpha-amino carboxylic acids, respectively) is not a compound according to (iv), more preferably is not a sulfur-containing amino carboxylic acid (preferably is not a sulfur-containing alpha-amino carboxylic acid, respectively), most preferably is not methionine. It is in particularly preferred that the one or more than one complexing agent is distinct from (iv).

More preferred is a method of the present invention, wherein the one or more than one complexing agent comprises formic acid, acetic acid, oxalic acid, tartaric acid, malic acid, citric acid, glycine, aspartic acid, and/or salts thereof, preferably formic acid, acetic acid, oxalic acid, tartaric acid, malic acid, citric acid, and/or salts thereof, more preferably formic acid, acetic acid, oxalic acid, tartaric acid, malic acid, and/or salts thereof, even more preferably formic acid, acetic acid, and/or salts thereof, most preferably formic acid and/or salts thereof.

Preferred is a method of the present invention, wherein the one or more than one complexing agent has a total concentration ranging from 5 g/L to 200 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably ranging from 8 g/L to 150 g/L, more preferably ranging from 10 g/L to 100 g/L, even more preferably from 12 g/L to 75 g/L, yet even more preferably ranging from 15 g/L to 50 g/L, most preferably ranging from 20 g/L to 35 g/L.

The aqueous trivalent chromium electroplating bath comprises (iii), i.e. colloidal particles.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is a colloidal suspension. This is due to the presence of said colloidal particles. However, preferably it is a very diluted colloidal suspension.

Preferred is a method of the present invention, wherein the colloidal particles comprise one or more than one chemical element selected from the group consisting of silicon, aluminum, and carbon, preferably silicon and aluminum, most preferably the colloidal particles comprise the chemical element aluminum.

Preferred is a method of the present invention, wherein the colloidal particles comprise nano-particles, preferably are nano-particles. Preferably, the colloidal particles have a particle size of less than 1000 nm, preferably of less than 500 nm, more preferably at least 90% of the colloidal particles have a particle size of less than 500 nm, most preferably at least 90% of the colloidal particles have a particle size of less than 150 nm.

Preferred is a method of the present invention, wherein said colloidal particles have an average particle diameter D₅₀ of 100 nm or less, preferably of 80 nm or less, more preferably of 60 nm or less, even more preferably of 50 nm or less, most preferably of 40 nm or less, very most preferably of 30 nm or less, even most preferably of 25 nm or less, based on volume.

More preferred is a method of the present invention, wherein said colloidal particles comprise at least particles with a particle size of 100 nm or less, preferably of 80 nm or less, more preferably of 60 nm or less, even more preferably of 50 nm or less, most preferably of 40 nm or less, very most preferably of 30 nm or less, even most preferably of 20 nm or less. Most preferably, a particle size of 100 nm is not exceeded.

Preferred is a method of the present invention, wherein the colloidal particles comprising the chemical element silicon, comprise silica, preferably are silica colloidal particles.

Preferred is a method of the present invention, wherein the colloidal particles comprising the chemical element aluminum, comprise aluminum oxide, preferably are aluminum oxide colloidal particles.

Most preferred is a method of the present invention, wherein said colloidal particles comprise aluminum oxide, preferably are aluminum oxide colloidal particles.

Preferred is a method of the present invention, wherein the colloidal particles comprising the chemical element carbon, comprise nanodiamonds, preferably are nanodiamond colloidal particles.

Preferred is a method of the present invention, wherein in step (b) said colloidal particles are present in a total amount ranging from 0.05 g/L to 100 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably from 0.1 g/L to 80 g/L, more preferably from 0.25 g/L to 60 g/L, even more preferably from 0.5 g/L to 45 g/L, most preferably from 0.75 g/L to 35 g/L, even most preferably from 1 g/L to 20 g/L.

The aqueous trivalent chromium electroplating bath comprises (iv), one or more than one sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below. Preferably, acids, salts, isoforms, and betaines thereof are included. However, sulfate anions are not counted among (iv).

In some cases, a method of the present invention is preferred, wherein said one or more than one sulfur-containing compound comprises a divalent sulfur atom.

Generally preferred is a method of the present invention, wherein in step (b) the aqueous trivalent chromium electroplating bath comprises (iv) in a total concentration ranging from 1 mmol/L to 1150 mmol/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably from 16 mmol/L to 900 mmol/L, more preferably from 30 mmol/L to 800 mmol/L, even more preferably from 70 mmol/L to 700 mmol/L, most preferably from 110 mmol/L to 595 mmol/L.

In some cases, preferred is a method of the present invention, wherein (iv) comprises at least one inorganic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below. However, in other cases a method of the present invention is preferred, wherein (iv) comprises at least one organic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below. In again other cases a method of the present invention is preferred, wherein (iv) comprises at least one inorganic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below and at least one organic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below.

Preferred is a method of the present invention, wherein (iv) comprises thiocyanate anions, preferably at least comprises thiocyanate anions. In the context of the present invention, thiocyanate anions (i.e. SCN⁻) are considered to be inorganic, wherein organic compounds comprising a thiocyanate moiety as a functional group are considered to be organic thiocyanates. Preferably, inorganic thiocyanate anions are present through a thiocyanate salt (e.g. potassium, sodium, ammonium thiocyanate) or through thiocyanic acid.

Particularly preferred is a method of the present invention, wherein in the aqueous trivalent chromium electroplating bath the thiocyanate anions have a total concentration ranging from 1 mmol/L to 400 mmol/L, based on the total volume of the electroplating bath, preferably from 3 mmol/L to 350 mmol/L, more preferably from 5 mmol/L to 300 mmol/L, even more preferably from 8 mmol/L to 250 mmol/L, yet even more preferably from 12 mmol/L to 200 mmol/L, most preferably from 15 mmol/L to 180 mmol/L.

Preferred is a method of the present invention, wherein (iv) comprises

-   -   at least an organic sulfur-containing compound having a sulfur         atom with an oxidation number of +5 or below and additionally         having a nitrogen atom, preferably an amino group;     -   more preferably (iv) comprises at least an amino acid having a         sulfur atom with an oxidation number of +5 or below;     -   most preferably (iv) comprises at least methionine.

In some cases, this preferably applies in addition to above-mentioned inorganic sulfur-containing compound, including preferably thiocyanate anions. However, in other cases, preferably it applies instead of above-mentioned inorganic sulfur-containing compound, including preferably thiocyanate anions. In the latter case, (iv) comprises only at least one of said organic sulfur containing compounds.

Preferably, the amino acid having a sulfur atom with an oxidation number of +5 or below comprises an alpha-amino acid having a sulfur atom with an oxidation number of +5 or below, most preferably a proteinogenic amino acid having a sulfur atom with an oxidation number of +5 or below. Most preferably this comprises methionine and/or cysteine.

Preferred is a method of the present invention, wherein in the aqueous trivalent chromium electroplating bath the organic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below and additionally having a nitrogen atom (preferably the at least an amino acid having a sulfur atom with an oxidation number of +5 or below, more preferably said alpha-amino acids) has a total concentration ranging from 10 mmol/L to 550 mmol/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably from 30 mmol/L to 480 mmol/L, more preferably from 60 mmol/L to 410 mmol/L, even more preferably from 80 mmol/L to 350 mmol/L, yet even more preferably from 100 mmol/L to 280 mmol/L, most preferably from 130 mmol/L to 200 mmol/L.

Particularly preferred is a method of the present invention, wherein in the aqueous trivalent chromium electroplating bath (iv) comprises

-   -   at least one inorganic sulfur-containing compound having a         sulfur atom with an oxidation number of +5 or below, preferably         thiocyanate anions,         and/or (preferably and)     -   at least an organic sulfur-containing compound having a sulfur         atom with an oxidation number of +5 or below and additionally         having a nitrogen atom, preferably an amino group; more         preferably an amino acid having a sulfur atom with an oxidation         number of +5 or below, most preferably methionine.

Thus, in some cases, a method of the present invention is preferred, wherein the aqueous trivalent chromium electroplating bath comprises two or more than two compounds of (iv), most preferably thiocyanate anions, methionine, acids, and/or salts thereof.

In some cases, a method of the present invention is even more preferred, wherein the aqueous trivalent chromium electroplating bath comprises one or more than one of the following compounds (including its salts):

-   (1) 2-(2-Hydroxy-ethylsulfanyl)-ethanol, -   (2) Thiazolidine-2-carboxylic acid, -   (3) Thiodiglycol ethoxylate, -   (4) 2-Amino-3-ethylsulfanyl-propionic acid, -   (5) 3-(3-Hydroxy-propylsulfanyl)-propan-1-ol, -   (6) 2-Amino-3-carboxymethylsulfanyl-propionic acid, -   (7) 2-Amino-4-methylsulfanyl-butan-1-ol, -   (8) 2-Amino-4-methylsulfanyl-butyric acid, -   (9) 2-Amino-4-ethylsulfanyl-butyric acid, -   (10) 3-Carbamimidoylsulfanyl-propane-1-sulfonic acid, -   (11) 3-Carbamimidoylsulfanyl-propionic acid, -   (12) Thiomorpholine, -   (13) 2-[2-(2-Hydroxy-ethylsulfanyl)-ethylsulfanyl]-ethanol, -   (14) 4,5-Dihydro-thiazol-2-ylamine, -   (15) Thiocyanic acid, -   (16) 2-Amino-4-methanesulfinyl-butyric acid, -   (17) 1,1-Dioxo-1,2-dihydro-1lambda*6*-benzo[d]isothiazol-3-one, -   (18) Prop-2-yne-1-sulfonic acid, -   (19) Methanesulfinylmethane, -   (20)     2-(1,1,3-Trioxo-1,3-dihydro-1lambda*6*-benzo[d]isothiazol-2-yl)-ethanesulfonic     acid

Preferably, above-mentioned compounds refer to (iv) in the aqueous trivalent chromium electroplating bath, most preferably compounds (1) to (16) and (19).

Preferably, the aqueous trivalent chromium electroplating bath comprises further compounds or preferably does not contain particular compounds as outlined in the following.

Preferred is a method of the present invention, wherein in step (b) the aqueous trivalent chromium electroplating bath further comprises Fe(II) ions, preferably in a concentration ranging from 0.1 mmol/L to 10 mmol/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably from 0.4 mmol/L to 8 mmol/L, more preferably from 0.8 mmol/L to 6 mmol/L, even more preferably from 1.2 mmol/L to 4 mmol/L, most preferably from 1.5 mmol/L to 2.5 mmol/L.

In many cases, said Fe(II) ions positively affect the electroplating performance. Furthermore, in some cases it is preferred that the chromium layer comprises iron.

Preferred is a method of the present invention, wherein in step (b) the aqueous trivalent chromium electroplating bath further comprises sulfate anions, preferably in a concentration ranging from 0.2 mol/L to 1.3 mol/L, based on the total volume of the aqueous trivalent chromium electroplating bath, more preferably from 0.3 mol/L to 1.1 mol/L, even more preferably from 0.4 mol/L to 1.0 mol/L, yet even more preferably from 0.5 mol/L to 0.9 mol/L, most preferably from 0.6 mol/L to 0.8 mol/L. Preferably, sulfate ions are present due to the source of trivalent chromium ions, e.g. basic chromium sulfate. Sulfate ions excellently contribute to the conductivity of said electroplating bath.

Preferred is a method of the present invention, wherein in step (b) the aqueous trivalent chromium electroplating bath further comprises halogen anions, preferably halogen anions in a total concentration ranging from 0.1 mol/L to 6 mol/L, based on the total volume of the aqueous trivalent chromium electroplating bath, more preferably in a total concentration ranging from 0.5 mol/L to 5 mol/L, even more preferably from 1 mol/L to 4.5 mol/L, yet even more preferably from 1.5 mol/L to 4.2 mol/L, most preferably from 2 mol/L to 3.9 mol/L.

More preferred is a method of the present invention, wherein the halogen anions comprise chloride anions, preferably in a total concentration ranging from 0.5 mol/L to 5 mol/L, based on the total volume of the aqueous trivalent chromium electroplating bath, more preferably from 0.8 mol/L to 4.7 mol/L, even more preferably from 1.3 mol/L to 4.5 mol/L, yet even more preferably from 1.8 mol/L to 4 mol/L, most preferably from 2.3 mol/L 3.7 mol/L. Chloride ions are preferably from a chloride salt and/or hydrochloric acid, preferably from sodium chloride, potassium chloride, ammonium chloride, chromium chloride (at least as a part of all chloride ions), and/or mixtures thereof. Typically, chloride ions are present as the anion of a conductivity salt. A very preferred conductivity salt is ammonium chloride, sodium chloride and potassium chloride, ammonium chloride being preferred most.

Preferred is a method of the present invention, wherein in step (b) the halogen anions comprise bromide anions, preferably either in addition or instead of chloride ions. Bromide ions typically avoid an anodic formation of undesired hexavalent chromium species. Preferably, the bromide ions have a concentration ranging from 3 g/L to 20 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably ranging from 4 g/L to 18 g/L, more preferably ranging from 5 g/L to 16 g/L, even more preferably ranging from 6 g/L to 14 g/L, most preferably ranging from 7 g/L to 12 g/L. Bromide ions are preferably from a bromide salt, preferably from sodium bromide, potassium bromide, ammonium bromide, and/or mixtures thereof. Preferably, the bromide ions are also present if sulfate ions are utilized in the aqueous trivalent chromium electroplating bath.

Preferred is a method of the present invention, wherein in step (b) the aqueous trivalent chromium electroplating bath further comprises ammonium ions.

Preferred is a method of the present invention, wherein in step (b) the aqueous trivalent chromium electroplating bath further comprises one or more than one pH buffer compound. Most preferably, the one or more than one pH buffer compound is distinct (i.e. different) from (ii) and (iv). This preferably means that the one or more than one pH buffer compound does not comprise a carboxylic acid, preferably does not comprise an organic acid.

In many cases a method of the present invention is preferred, wherein in the aqueous trivalent chromium electroplating bath the one or more than one pH buffer compound comprises a boron-containing compound, preferably boric acid and/or a borate, most preferably boric acid. A preferred borate is sodium borate.

Generally preferred is method of the present invention, wherein in the aqueous trivalent chromium electroplating bath the one or more than one pH buffer compound has a total concentration ranging from 30 g/L to 250 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably ranging from 35 g/L to 200 g/L, more preferably ranging from 40 g/L to 150 g/L, even more preferably ranging from 45 g/L to 100 g/L, most preferably ranging from 50 g/L to 75 g/L. This even more preferably applies to said boron-containing compound, yet even more preferably to said boric acid together with said borate, most preferably to boric acid. Most preferably the one or more than one pH buffer compound comprises boric acid but no borate. Thus, most preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath comprises boric acid, preferably in a total concentration ranging from 35 g/L to 90 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath, preferably from 40 g/L to 80 g/L, more preferably from 50 g/L to 70 g/L, most preferably from 56 g/L to 66 g/L.

However, in some other cases the aqueous trivalent chromium electroplating bath does not explicitly comprise a distinct pH buffer compound. Rather, the one or more than one complexing agent for said trivalent chromium ions are present in such an amount and selected in such a way that they do not only serve as complexing agent for the trivalent chromium ions but additionally serve as pH buffer compound. In the context of the present invention this is less preferred but possible.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, ions and/or compounds comprising zinc. Preferably, the dark chromium layer is substantially free of, preferably does not comprise, zinc.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is not a conversion treatment composition. In other words, the aqueous trivalent chromium electroplating bath is not suitable for conversion coatings and/or for applying on a zinc or zinc alloy layer. In yet even other words, the method of the present invention is not a conversion coating method.

Preferred is a method of the present invention, wherein the substrate is substantially free of, preferably does not comprise, a zinc and zinc alloy layer.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, fluoride ions. Preferably, the dark chromium layer is substantially free of, preferably does not comprise, fluorine.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, phosphate anions, more preferably is substantially free of, preferably does not comprise, phosphorous-containing compounds. Preferably, the chromium layer is substantially free of, preferably does not comprise, phosphorous.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, sulfite anions.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, a compound comprising chromium with an oxidation number +6. Thus, said electroplating bath is substantially free of, preferably does not comprise, hexavalent chromium. This in particular means that hexavalent chromium is at least not intentionally added to the aqueous trivalent chromium electroplating bath. However, this does not exclude anodically, unavoidably formed hexavalent chromium in insignificant amounts.

In some cases, preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, ions and/or compounds comprising cobalt. Preferably, the chromium layer is substantially free of, preferably does not comprise, cobalt. However, in other cases a method of the present invention is preferred, wherein the aqueous trivalent chromium electroplating bath comprises ions and/or compounds comprising cobalt. Preferably, the chromium layer comprises in such a case cobalt. Although cobalt is environmental questionable, it provides a darkening effect in some cases, preferably an additional darkening effect.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, a soluble aluminum compound (including salts thereof), preferably does not comprise dissolved aluminum ions.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, nickel ions. In some cases, a typical Ni-contamination of up to 150 ppm is observed, which is basically acceptable and therefore considered as substantially free of nickel ions. Thus, in some cases it is preferred that the nickel ions have a concentration ranging from 0 ppm to 200 ppm, based on the total weight of the aqueous trivalent chromium electroplating bath, preferably from 1 ppm to 150 ppm, most preferably from 2 ppm to 100 ppm. However, most preferably the aqueous trivalent chromium electroplating bath is free of nickel.

It is generally preferred to avoid environmental questionable nickel and cobalt species. This generally leads to less complicated wastewater treatment and bath disposal. In addition, neither nickel nor cobalt is necessarily needed to obtain a dark chromium layer.

Preferred is a method of the present invention, wherein the aqueous trivalent chromium electroplating bath is substantially free of, preferably does not comprise, a sulfamic acid and salts thereof.

In step (c) of the method of the present invention, the substrate is contacted with the aqueous trivalent chromium electroplating bath and an electrical current is applied such that the dark chromium layer is electrolytically deposited on the substrate and is fully covering at least one side of the substrate. Preferably, the dark chromium layer comprises a dark chromium alloy layer. In other words, the dark chromium layer in the context of the present invention preferably refers likewise to a dark chromium alloy layer. Preferably, features throughout the present text preferably apply likewise to the dark chromium alloy layer.

Preferred is a method of the present invention, wherein the electrical current in step (c) is a direct current, preferably in a range from 3 A/dm² to 30 A/dm², more preferably from 4 A/dm² to 25 A/dm², even more preferably from 5 A/dm² to 20 A/dm², most preferably from 6 A/dm² to 18 A/dm².

Preferred is a method of the present invention, wherein in step (c) at least one anode is provided and utilized. The at least one anode is preferably selected from the group consisting of graphite anodes, precious metal anodes, and mixed metal oxide anodes (MMOs).

Preferred precious metal anodes comprise platinized titanium anodes and/or platinum anodes.

Preferred mixed metal oxide anodes comprise platinum oxide coated titanium anodes and/or iridium oxide coated titanium anodes.

Preferred is a method of the present invention, wherein the chromium layer electrolytically deposited in step (c) has a layer thickness ranging from 0.05 μm to 1 μm, preferably from 0.1 μm to 0.8 μm, more preferably from 0.125 μm to 0.6 μm, most preferably from 0.15 μm to 0.5 μm.

Preferred is a method of the present invention, wherein in step (c) the contacting is carried out for 1 minute to 20 minutes, preferably for 2 minutes to 15 minutes, more preferably from 3 minutes to 10 minutes.

Preferred is a method of the present invention, wherein in step (c) the contacting is carried out at a temperature ranging from 20° C. to 60° C., preferably from 25° C. to 52° C., more preferably from 30° C. to 45° C.

In step (c) of the method of the present invention a dark chromium layer is electrolytically deposited (i.e. it is an electroplated dark chromium layer) on the substrate with a lightness value L* according to the L*a*b* color-space system as well as an a* and b* value; in particular the dark chromium layer fully covering at least one side of the substrate. Typically, the L*, a*, and b* values slightly vary such that on said at least one side fully covered by the dark chromium layer has a highest and lowest L* value (L2* and L1*, respectively), a highest and lowest a* value (a2* and a1*, respectively) and highest and lowest b* value (b2* and b1*, respectively). The closer each highest and lowest value are together, the more homogeneous is the overall color perception.

Preferred is a method of the present invention, wherein the dark chromium layer is preferably black. This means it is preferably an electroplated black chromium layer. This preferably also applies to the substrate of the present invention (see text below).

Preferred is a method of the present invention, wherein the dark chromium layer fully covering said at least one side has L* values, according to the L*a*b color system, ranging from 30 to 55, preferably from 33 to 53, more preferably from 35 to 51, even more preferably from 37 to 50, yet even more preferably from 39 to 49, most preferably from 40 to 47, yet even most preferably from 41 to 45.

More preferred is a method of the present invention, wherein the dark chromium layer fully covering said at least one side has a highest L* value L2*, according to the L*a*b color system, of 55 or less, preferably 53 or less, more preferably 51 or less, even more preferably 50 or less, yet even more preferably 49 or less, most preferably 47 or less, yet even most preferably 45 or less.

Preferred is a method of the present invention, wherein the dark chromium layer fully covering said at least one side has a* values ranging from −1.5 to +3, preferably from −1 to +2.5, most preferably from −0.5 to +2.

More preferred is a method of the present invention, wherein the dark chromium layer fully covering said at least one side has a highest a2* value of +3 or less, preferably of +2.5 or less, more preferably of +2 or less, most preferably of +1.5 or less. Most preferably, the highest a2* values are at least positive.

Preferred is a method of the present invention, wherein the dark chromium layer fully covering said at least one side has b* values ranging from −1.5 to +6, preferably from −1.2 to +5, more preferably from −0.9 to +4, even more preferably from −0.7 to +3, yet even more preferably from −0.5 to +2.7, most preferably from −0.3 to +2.5, yet even most preferably from 0 to +2.2.

More preferred is a method of the present invention, wherein the dark chromium layer fully covering said at least one side has a highest b* value b2*, according to the L*a*b color system, of +6 or below, preferably of +5 or below, more preferably of +4 or below, even more preferably of +3 or below, most preferably of +2 or below.

Preferred is a method of the present invention further comprising prior to step (c) at least one metal plating step to deposit at least one metal or metal alloy layer on the substrate, most preferably at least one nickel plating step to deposit at least one nickel and/or nickel alloy layer. In many cases two or even three such metal plating steps are preferably involved.

Most preferably, the at least one nickel and/or nickel alloy layer comprises at least one bright-nickel layer and/or (preferably or) at least one satin nickel layer, most preferably at least one bright-nickel layer.

More preferred is a method of the present invention, wherein the at least one nickel and/or nickel alloy layer comprises at least one semi-bright nickel layer, preferably at least one semi-bright-nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer. The at least one semi-bright nickel layer is preferably optionally. Most preferably (if applied) the at least one semi-bright nickel layer is deposited prior to said at least one bright-nickel layer and/or said at least one satin nickel layer.

Also preferred is a method of the present invention, wherein the at least one nickel and/or nickel alloy layer comprises at least one MPS nickel layer (i.e. a non-conductive particle-containing nickel layer), preferably at least one MPS nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer, most preferably at least one MPS nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer, and further to said at least one semi-bright nickel layer. In the context of the present invention MPS denotes that the MPS nickel layer comprises non-conductive micro-particles, which cause micro-pores in the dark chromium layer. The at least one MPS nickel layer is preferably optionally.

In some cases, a method of the present invention is preferred, wherein the MPS nickel layer is adjacent to the dark chromium layer.

In other cases, a method of the present invention is preferred wherein the dark chromium layer is adjacent to the at least one bright-nickel layer and/or the at least one satin nickel layer, which is in many cases preferred, most preferably in combination with the at least one bright-nickel layer.

Preferably, the dark chromium layer, is part of a layer stack.

In step (d) of the method of the present invention the substrate obtained from step (c) is treated with a rinse liquid, wherein the rinse liquid has a temperature of 50° C. or more and wherein during step (d) no electrical current is applied. Thus, step (d) is an electroless step and most preferably a mere rinse step. In other words, preferably no external electrical current is applied as well as preferably no chemical reducing agent is involved in this step. Furthermore, steps (c) and (d) are distinct steps, i.e. individual steps, subsequently carried out, first step (c) followed by step (d).

In the context of the present invention, step (d) is a very important step because it allows a quick and direct formation of the desired dark color tone of the dark chromium layer and is considered to be as an accelerated aging step. The temperature applied in step (d) is not the temperature utilized in step (c) but preferably the temperature utilized in step (d) is higher than the temperature utilized in step (c).

Preferred is a method of the present invention, wherein the treating in step (d) is carried out at a temperature ranging from 55° C. to 99° C., preferably from 60° C. to 95° C., more preferably from 63° C. to 91° C., even more preferably from 66° C. to 88° C., yet even more preferably from 69° C. to 85° C., most preferably from 72° C. to 83° C.

Preferred is a method of the present invention, wherein in step (d) the rinse liquid comprises water, i.e. is preferably aqueous, preferably substantially is only water. Most preferably the rinse liquid consists of water.

More preferred is a method of the present invention, wherein in step (d) the treating with the rinse liquid is an immersion into the rinse liquid. However, in some cases a method of the present invention is preferred, wherein in step (d) the treating with the rinse liquid comprises a contacting with a flow of the rinse liquid, preferably a constant flow of the rinse liquid.

Preferred is a method of the present invention, wherein the treating in step (d) is carried out for a period of 2 minutes to 90 minutes, preferably from 3 minutes to 75 minutes, more preferably from 4 minutes to 60 minutes, even more preferably from 6 minutes to 50 minutes, most preferably from 8 minutes to 40 minutes, even most preferably from 10 minutes to 35 minutes.

The method of the present invention preferably does not exclude further steps, preferably such as additional rinsing, cleaning, pre-treating, and/or post-treating steps. Preferably, steps defined in the examples below apply likewise to the general method described throughout the present text. A preferred post-treating step comprises a sealing step, preferably with an inorganic and/or organic sealer, and/or a contacting step with an anti-fingerprint composition.

The present invention furthermore relates to a specifically defined substrate comprising a dark chromium layer at least fully covering one side of the substrate. The present invention in particular refers to a substrate having at least one side fully covered with a dark chromium layer, wherein said dark chromium layer fully covering said at least one side has

-   -   a lowest L* value L1* and a highest L* value L2*,     -   a lowest a* value a1* and a highest a* value a2*, and     -   a lowest b* value b1* and a highest b* value b2*,         -   wherein according to         -   ΔE=√{square root over ((L2*−L1*)²+(a2*−a1*)²+(b2*−b1*)²)},             ΔE is ranging from 0.0001 to 3.5, based on said dark             chromium layer fully covering said at least one side,             preferably from 0.0001 to 3, more preferably from 0.0001 to             2.5, even more preferably from 0.0001 to 2.3, most             preferably from 0.0001 to 2, and     -   L2* is 50 or less.

The aforementioned regarding the method of the present invention preferably applies likewise to the specifically defined substrate of the present invention (if technical applicable). This most preferably applies to the L*a*b* values and layer thickness defined for the method of the present invention. However, the following features of the substrate of the present invention preferably also apply to the method of the present invention (if not already mentioned in the text above).

Preferred is a substrate of the present invention, wherein L2* is 49 or less, preferably 48 or less, more preferably 47 or less, even more preferably 46 or less, yet even more preferably 45 or less, most preferably 44 or less, yet even most preferably 43 or less.

Preferred is a substrate of the present invention, wherein b2* is +7 or less, preferably +6 or less, more preferably +5 or less, even more preferably +4 or less, most preferably +3 or less.

Preferred is a substrate of the present invention, wherein the dark chromium layer comprises aluminum and/or silicon, preferably aluminum.

More preferred is a substrate of the present invention, wherein the dark chromium layer comprises particles, preferably particles comprising aluminum and/or silicon, most preferably particles comprising aluminum.

Preferably, the dark chromium layer is not the only metal layer between the dark chromium layer and the substrate.

Preferred is a substrate of the present invention, wherein the substrate comprises in addition to the dark chromium layer at least one nickel and/or nickel alloy layer under the dark chromium layer.

Preferred is a substrate of the present invention, wherein said at least one nickel and/or nickel alloy layer comprises at least one bright-nickel layer or at least one satin nickel layer. This is most preferred.

Preferred is a substrate of the present invention, wherein said at least one nickel and/or nickel alloy layer comprises at least one semi-bright nickel layer, preferably at least one semi-bright-nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer. The at least one semi-bright nickel layer is preferably optionally. Most preferably the at least one semi-bright nickel layer is the respective nickel and nickel alloy layer, respectively, of all nickel and nickel alloy layers closest to the substrate.

Preferred is a substrate of the present invention, wherein said at least one nickel and/or nickel alloy layer comprises at least one MPS nickel layer, preferably at least one MPS nickel layer in addition to said at least one bright-nickel layer and/or said at least one satin nickel layer, most preferably at least one MPS nickel layer in addition to said at least one bright-nickel layer, and/or said at least one satin nickel layer, and further to said at least one semi-bright nickel layer. In the context of the present invention MPS denotes micro porous.

Preferred is a substrate of the present invention, wherein said at least one MPS nickel layer faces on the one side the dark chromium layer and on the other side said at least one bright-nickel layer or said at least one satin nickel layer.

Preferred is a substrate of the present invention, wherein said at least one semi-bright nickel layer is under said at least one bright-nickel layer or said at least one satin nickel layer (defined from a top view towards the substrate).

Preferred is a substrate of the present invention, wherein the dark chromium layer is part of a layer stack, the layer stack comprising, defined from a top view towards the substrate (adjacent or not adjacent):

-   -   (i) the dark chromium layer,     -   (ii) optionally, at least one MPS nickel layer,     -   (iii) at least one bright-nickel layer and/or (preferably or) at         least one satin nickel layer, and     -   (iv) optionally, at least one semi-bright nickel layer.

In some cases, the layer stack preferably comprises a sealer layer and/or an anti-fingerprint layer, most preferably on top of the dark chromium layer. If both is applied, preferably the sealer layer is applied first, followed by the anti-fingerprint layer, which preferably forms the very outermost layer.

The spirit of the present invention is further illustrated in the following examples without limiting the scope of the invention as herein defined in the claims.

Examples (a) Providing the Substrate:

For the following examples copper panels (100 mm×70 mm) were used as a substrate, primarily for mimicking a plastic substrate with a copper layer thereon.

In a first step the substrate was cleaned by electrolytic degreasing with Uniclean® 279 (product of Atotech), 100 g/L at room temperature (RT). Afterwards the copper panels were pickled with 10 vol.-% H₂SO₄ and subsequently rinsed with water.

In a second step, the cleaned and rinsed substrate was subjected to nickel plating to obtain a bright nickel layer on top of the copper panel (parameters: 10 min at 4 A/dm²; UniBrite 2002, product of Atotech).

(b) Providing the Aqueous Trivalent Chromium Electroplating Bath

The following base aqueous trivalent chromium electroplating bath was used:

128 g/L Basic chromium sulfate 46 g/L Formic acid 60 g/L Boric acid 12 g/L Ammonium bromide 100 g/L Ammonium chloride 110 g/L Potassium chloride 30 g/L Methionine 15 g/L Potassium thiocyanate 2 mmol/L Fe(II) ions

The final pH value was 3.5.

Additional amounts of colloidal particles and kinds thereof are summarized in Table 1 below. “CE” denotes a comparative example, wherein “E” denotes an example according to the invention.

TABLE 1 further aqueous trivalent chromium electroplating bath parameters Colloidal Particles No. element/kind c [ml/L] size D₅₀ [nm] CE1 n/a n/a n/a E1 Si/silica (SiO₂)^(#) 2 20 E2 Si/silica (SiO₂)^(#) 4 20 E3 Al/alumina (Al₂O₃)^(##) 2 25 CE2* n/a n/a n/a E4* Al/alumina (Al₂O₃)^(##) 2 25 E5* Si/silica (SiO₂)^(#) 4 20 *containing additional 10 g/L potassium thiocyanate ^(#)dispersion, 40% (Silicon(IV) Oxide, Alfa Aesar) ^(##)dispersion, 40% (NANOBYK-3603, BYK-Chemie GmbH) (c) Contacting the Substrate with Said Electroplating Bath

Electroplating was carried out in a Hull Cell having a graphite anode and the substrate installed as the cathode. An electrical current of 5 A was applied to the aqueous trivalent chromium electroplating bath for 3 minutes, the bath having a temperature of about 35° C. Agitation was achieved via air agitation. As a result, a respective dark electroplated chromium layer was fully deposited on a specifically defined optical area of the nickel-plated copper panel, and therefore mimicking a typical plastic substrate, which would be otherwise fully covered on its front side.

(d) Treating the Substrate with a Rinse Liquid

Afterwards, the substrate having the dark chromium layer was rinsed with hot water (80° C.) in a rinse step for 30 minutes by immersing the substrate into a beaker comprising said water.

After the rinse step, substrates were stored for 15 days to achieve a certain color equilibrium. Afterwards, L*a*b* values, according to the L*a*b* color-space system, were determined (Konica Minolta CM-700 D spectrophotometer; CIE standard illuminator D65 and 10° standard observer) on a variety of locations on the optical side. Calibration of the spectrophotometer was done with black and white standards.

L*a*b* values were determined in a range from 3.5 cm from the left edge of the substrate (and 2 cm from the lower edge of the substrate; the left edge pointing to the anode) to 1 cm from the left edge. In this area, typically the highest and lowest L*a*b* values over the entire optical side are determined. Thus, the highest and lowest values, respectively, were determined and are summarized in Table 2 below.

TABLE 2 Summary of color values and ΔE L* a* b* No. L1* L2* a1* a2* b1* b2* ΔE CE1 46.37 46.42 1.36 1.76 4.01 6.50 2.5 E1 45.42 46.84 1.72 2.07 4.52 5.70 1.9 E2 46.74 47.86 0.96 1.35 5.21 6.40 1.7 E3 42.99 45.19 1.35 1.37 6.01 6.62 2.3 CE2 42.20 43.45 0.46 1.73 −0.27 4.28 4.9 E4 41.70 41.78 0.99 1.06 0.85 2.91 2.1 E5 38.87 39.69 0.40 0.88 1.03 3.82 3.0

Table 2 shows that for all examples according to the invention (i.e. E1 to E5) ΔE is reduced compared to its respective comparative example (i.e. CE1 and CE2, respectively). Thus, the over-all color impression, defined by L*, a*, and b* is more homogeneous.

In particular E4 shows very desirable b* values of less than 3, combined with a comparatively low ΔE.

Furthermore, examples E4 and E5 show that a comparatively higher concentration of thiocyanate anions results in generally lower b* values compared to examples E1 to E3 and independently of the kind of particles used. This preferably also applies to the brightness L*.

In addition, example 4 is the only example reaching a very desirable b* value of less than +3, which very closely relates to a neutral black color tone.

Typically, a ΔE of more than 3.5 (see CE2) is mostly not desired. 

1. A method for electrodepositing a dark chromium layer on a substrate, the method comprising the steps (a) providing a substrate, (b) providing an aqueous trivalent chromium electroplating bath comprising (i) trivalent chromium ions, (ii) one or more than one complexing agent for said trivalent chromium ions, (iii) colloidal particles, and (iv) one or more than one sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below, (c) contacting the substrate with said electroplating bath and applying an electrical current such that the dark chromium layer is electrolytically deposited on the substrate and is fully covering at least one side of the substrate, (d) treating the substrate comprising the electrolytically deposited dark chromium layer with a rinse liquid, wherein the rinse liquid has a temperature of 50° C. or more and wherein during step (d) no electrical current is applied.
 2. The method of claim 1, wherein the colloidal particles comprise one or more than one chemical element selected from the group consisting of silicon, aluminum, and carbon.
 3. The method of claim 1, wherein said colloidal particles comprise nano-particles.
 4. The method of claim 1, wherein said colloidal particles comprise at least particles with a particle size of 100 nm or less.
 5. The method of claim 2, wherein the colloidal particles comprising the chemical element silicon, comprise silica.
 6. The method of claim 2, wherein the colloidal particles comprising the chemical element aluminum, comprise aluminum oxide.
 7. The method of claim 2, wherein the colloidal particles comprising the chemical element carbon, comprise nanodiamonds.
 8. The method of claim 1, wherein in step (b) said colloidal particles are present in a total amount ranging from 0.05 g/L to 100 g/L, based on the total volume of the aqueous trivalent chromium electroplating bath.
 9. The method of claim 1, wherein (iv) comprises thiocyanate anions.
 10. The method of claim 1, wherein (iv) comprises at least an organic sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below and additionally having a nitrogen atom.
 11. The method of claim 1, wherein the dark chromium layer fully covering said at least one side has a highest L* value L2*, according to the L*a*b color system, of 55 or less.
 12. The method of claim 1, wherein the dark chromium layer fully covering said at least one side has a highest b* value b2*, according to the L*a*b color system, of +6 or below.
 13. A substrate having at least one side fully covered with a dark chromium layer, wherein said dark chromium layer fully covering said at least one side has a lowest L* value L1* and a highest L* value L2*, a lowest a* value a1* and a highest a* value a2*, and a lowest b* value b1* and a highest b* value b2*, wherein according to ΔE=√{square root over ((L2*−L1*)²+(a2*−a1*)²+(b2*−b1*)²)}, ΔE is ranging from 0.0001 to 3.5, based on said dark chromium layer fully covering said at least one side, and L2* is 50 or less.
 14. The substrate of claim 13, wherein b2* is +7 or less.
 15. The substrate of claim 13, wherein the dark chromium layer comprises aluminum and/or silicon.
 16. The method of claim 1, wherein in step (d) the rinse liquid consists of water.
 17. The method of claim 1, wherein, in step (c) the contacting is carried out at a temperature ranging from 20° C. to 60° C., and in step (d) the rinse liquid has a temperature greater than the temperature in step (c). 